Vaisala WXT520 - Manual (Page 160)

User’s Guide ______________________________________________________________________
158 _________________________________________________________________ M210906EN-C
SDI-12 Communications Protocol
SDI-12 data recorders and sensors communicate by an exchange of
ASCII characters on the data line. The data recorder sends a break to
wake up the sensors on the data line. A break is continuous spacing on
the data line for at least 12 milliseconds. The data recorder then sends a
command. The sensor, in turn, returns the appropriate response. Each
command is for a specific sensor. The first character of each command
is a unique sensor address that specifies with which sensor the recorder
wants to communicate. Other sensors on the SDI-12 bus ignore the
command and return to low-power standby mode. When a data recorder
tells a sensor to start its measurement procedure, the recorder does not
communicate with any other sensor until the data collection from the
first sensor is complete.
A typical recorder/sensor measurement sequence proceeds in the
following order:
1. The data recorder wakes all sensors on the SDI-12 bus with a
break.
2. The recorder transmits a command to a specific, addressed sensor,
instructing it to make a measurement.
3. The addressed sensor responds within 15.0 milliseconds returning
the maximum time until the measurement data will be ready and
the number of data values it will return.
4. If the measurement is immediately available, the recorder transmits
a command to the sensor instructing it to return the
measurement(s). If the measurement is not ready, the data recorder
waits for the sensor to send a request to the recorder, which
indicates that the data is ready. The recorder then transmits a
command to get the data.
5. The sensor responds, returning one or more measurements.